1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a method and apparatus for efficiently selecting and acquiring a preferred wireless communications system.
2. Description of the Related Art
A mobile device will often have access to more than one wireless communications system in its geographic region. The quality of the wireless services available to the mobile device may vary from system to system depending on the equipment used by each system, the features of the mobile device, the distance between the mobile device and local base stations, physical obstructions such as buildings and hills, and the volume of communications traffic on each system. The wireless communications systems may also support different multiple-access wireless communications protocols such as code division multiple access (CDMA), wideband CDMA (WCDMA), Advanced Mobile Phone Service (AMPS), Global System for Mobile communications (GSM), General Packet Radio Services (GPRS) or High Data Rate (HDR) technology (e.g., 1xEV technology). In addition, the fees charged to a user of the mobile device may vary depending on the time of day, the duration of the connection with the wireless communications system and whether the mobile device is listed as a subscriber of the wireless communications system.
To assist with the selection of a desirable wireless communications system, conventional mobile devices store data describing known systems in a preferred roaming list (PRL). The PRL typically includes a systems table which stores a system identifier (SID) and a network identifier (NID) for each known wireless communications system, and an acquisition table which stores acquisition parameters including band, frequency and mode, for the known wireless communications systems. Within the systems table, the wireless communications systems may be grouped by geographic region and sorted from the most desirable to the least desirable system in each region. The most desirable system in a particular geographic region is typically a subscriber system, but may also be a roaming system that offers the mobile device a favorable combination of low cost and high quality of service. Roaming systems offer wireless services to non-subscriber mobile devices, usually at a much higher fee than subscription services, and may be desirable when the mobile device enters a geographic region that is outside the coverage area of the mobile device's subscription service, when the subscription services are blocked or otherwise unavailable, or when the subscription services are only available at an unacceptably low level of quality.
During a mobile device's power-up sequence, attempts are made to acquire and register with the most desirable wireless communications system that is available to the mobile device in its current geographic region. In one approach, the mobile device identifies its current geographic region and then steps through the entries in the systems table, from the most desirable system in the identified geographic region to the least desirable system in the identified geographic region, until a system acquisition and registration attempt is successful. The mobile device may also attempt to acquire and register with a new wireless communications system during operation. For example, the connection between the mobile device and a current wireless communications system may be lost, requiring the acquisition of a new system by the mobile device. In addition, the wireless communications systems that are available to the mobile device may change as the position of the mobile device and its surrounding environment changes. To provide the user of the mobile device with the best combination of high quality and low cost, many mobile devices periodically attempt to acquire and register with a wireless communications system that is more desirable than the current wireless communications system used by the mobile device. The mobile device typically searches the systems table for wireless communications systems in its geographic region that are more desirable than the current system used by the mobile device. If more desirable systems are found in the systems table, the mobile device will switch away from the current communications channel and attempt to acquire and register with one of the more desirable systems.
A system acquisition sequence, such as the system acquisition sequences described above, will often include a series of failed attempts to acquire a signal and register with the corresponding wireless communications system, followed by a single successful system acquisition and registration. These failed registration/acquisition attempts are common, time consuming (e.g., each failed attempt may take up to 20 seconds) and may be caused by a variety of factors. For example, the mobile device may be unable to detect a pilot signal transmitted from a system base station if the pilot signal is blocked or weakened by physical obstructions, or if the mobile device is outside the base station's coverage area. Registration with the system may fail if the mobile device and the system use incompatible protocol revisions and hardware, or if system otherwise rejects the registration attempt of the mobile device.
In view of the above, there is a need in the art for a method and apparatus for efficiently selecting and acquiring a wireless communications system.